Until quite recently, honeycomb-type fluid conduits, or catalytic converter elements, have been formed of ceramic monoliths as cast bodies having a plurality of longitudinally extending chambers forming a honeycomb-like structure. A catalytic material or composition is deposited on the walls of the chambers, and as the fluid or gas passes through at exhaust temperatures, a catalyzed reaction occurs whereby harmful pollutants contained within the gas stream, e.g., unburned hydrocarbons, carbon monoxide, ozone, nitrogen oxides, etc., are largely converted to harmless gases, e.g., water, carbon dioxide, and nitrogen, and passed into the air.
The ceramic type catalytic converters, although currently widely used, have limitations that can be overcome with corrugated metal foil substrates. In the first place, in order to preserve strength, and to provide adequate surface area for the catalyst, the size of the monolith must be quite large. Automotive customers are caught in the squeeze between this size requirement and the minimum ground clearance that must be preserved in order to minimize danger from grass fire. The ceramic monoliths are also fragile and careful handling is required. Thus, it can be seen that there is a great deal of interest in catalytic converters made from corrugated ferrous metal strips. These structures are strong and with openings numbering as high as 500 per square inch, the size requirements can be such that the automotive manufacturers have much more latitude in design. Ceramic honeycombs because of the thicker cell walls have about 70% open area compared to about 90% with the metallic honeycombs. This difference means a considerably high pressure drop in the ceramic honeycombs compared to the metallic honeycombs.
These metal support catalytic converters work very well with spark ignited internal combustion engines. However, compression ignited engines pose a somewhat different problem--that of particulate emissions composed mainly of carbon. A great deal of attention is now being directed to the solution of this problem, and it is to this end that the present invention is particularly directed.
With the number of openings in the range of from 250 to 500 per square inch, and a nonnesting corrugated pattern employed, the particulates collect within the converter and unless frequently burned out, will clog the openings and either shut down the engine or cause the exhaust to be by-passed through a noncatalyst exit. To minimize clogging, it has been found desirable to reduce the density to a maximum of about 200 cells per square inch. However, when such a reduction is made, the corrugations must be larger and clogging is avoided, but it is not possible to manufacture nonnesting configurations of the metal foil. Straight through openings are, therefore, provided. This structure necessitates the use of two foil strips, a noncorrugated foil strip and a corrugated strip in juxtaposed relation. Usually, the two strips are spirally wound.
A suitable example of a basic structure of the type here contemplated is shown in the patent to Rosenberger, U.S. Pat. No. 4,300,956 dated Nov. 17, 1981. The invention in this patent relates to a means of securing the crests of the corrugations to the adjacent noncorrugated strip by metal-to-metal diffusion bonding. The present invention effects bonding by a different means with unusual advantages. This invention uses a ceramic cement as will be described below.
As indicated above, the structures of the present invention may also carry a catalytically active surface for effecting chemical reactions while the fluid is traversing the fluid conduit. Reference may be had to my copending application Ser. No. 830,698 filed Feb. 18, 1986 for description of a general process for forming and rendering catalytically active a corrugated metal strip. Although the corrugations in that patent application are of the nonnesting type and at a density higher than in the present case, the principles of fabrication and depositing the catalyst on a suitably receptive surface are essentially the same. Reference may also be had to application Ser. No. 029,661 filed Mar. 24, 1987 entitled "Catalyst Support and Method for Making Same" for a preferred method of coating the base metal with aluminum.